1. Field of the Invention
The present invention relates to a control system and method for an internal combustion engine of an in-cylinder injection type, which causes the engine to operate while switching the combustion mode of the engine between a stratified combustion mode and a homogeneous combustion mode and controls a fuel injection amount based on a calculated demanded fuel amount.
2. Description of the Related Art
Conventionally, a control system of the above-mentioned kind has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 11-50895. According to this control system, even after the combustion mode of the engine installed on a vehicle is switched from the stratified combustion mode to the homogeneous combustion mode, some of a plurality of cylinders continue stratified combustion. In this case, the fuel injection amount for the cylinders that continue stratified combustion is set to a value calculated by multiplying the fuel injection amount calculated in the immediately preceding loop, a ratio between the present and immediately preceding values of a torque correction amount set based on the rotational speed of the engine, and a ratio between the present and immediately preceding values of a basic fuel injection amount. More specifically, the torque correction amount is set to a value increased or decreased by a fixed amount depending on which is larger, the rotational speed of the engine or a traget value thereof. The basic fuel injection amount is set to a weighted average of its immediately preceding value and the current provisional value calculated based on the amount of intake air supplied to the engine and the rotational speed of the engine. By the calculation described above, the fuel injection amount is controlled such that changes in the rotation speed of the engine and the amount of intake air are reflected therein, and at the same time an operation load on the control system is reduced.
However, the conventional control system described above suffers from the following problems: In the above method of calculating the fuel injection amount, when the combustion mode is switched, the fuel injection amount is calculated based on the immediately preceding value thereof, i.e. by multiplying the immediately preceding value by the ratio between the current and immediately preceding values of the torque correction amount and the ratio between the current and immediately preceding values of the basic fuel injection amount. However, since the torque correction amount is merely increased or decreased by the fixed amount depending on which is larger, the rotational speed of the engine speed or the target value thereof, as described above, the ratio between the current and immediately preceding values of the torque correction amount does not correctly reflect a change in torque demanded of the engine dependent on a change in the rotational speed of the engine. Further, since the basic fuel injection amount is a weighted average of the immediately preceding value and the current provisional value set according to the amount of intake air and the rotational speed of the engine, in short, an avaraged value, the ratio between the current and immediately preceding values of the basic fuel injection amount do not correctly reflect a change in the torque based on the amount of intake air and the rotational speed of the engine, either. Therefore, the output torque of the engine obtained from the fuel injection amount calculated based on these parameters fails to agree with the demanded torque, which causes degradation of drivability.
In the above-described engine, assuming that fuel cut-off operation was exeucted in parallel with switching of the combustion mode, and that after termination of the fuel cut-off operation, the vehicle is attempted to be accelerated, the immediately preceding value of the fuel injection amount is set to 0 due to the fuel cut-off operation, and the fuel injection amount currently calculated as described avove and applied to the part of the cylinders also assumes a value of 0, which prevents torque from being obtained from the above-mentioned part of the cylinders, resulting in shortage of the torque of the engine as a whole.
Further, this control system provides the above-described control on the fule injection amount when the combustion mode is switched from the stratified combustion mode to the homonogeneous combustion but does not provide control when the combustion mode is switched in an opposite direction. In such a case, it is impossible to properly control the output torque, which also leads to degraded drivability.